Fix incomplete URL substring sanitation (#7056)

Resolves code scanning alert in https://github.com/ultralytics/yolov5/issues/7055

utils/datasets.py

@@ -15,6 +15,7 @@ from itertools import repeat
 from multiprocessing.pool import Pool, ThreadPool
 from pathlib import Path
 from threading import Thread
+from urllib.parse import urlparse
 from zipfile import ZipFile
 
 import cv2
@@ -301,7 +302,7 @@ class LoadStreams:
         for i, s in enumerate(sources):  # index, source
             # Start thread to read frames from video stream
             st = f'{i + 1}/{n}: {s}... '
-            if 'youtube.com/' in s or 'youtu.be/' in s:  # if source is YouTube video
+            if urlparse(s).hostname in ('youtube.com', 'youtu.be'):  # if source is YouTube video
                 check_requirements(('pafy', 'youtube_dl==2020.12.2'))
                 import pafy
                 s = pafy.new(s).getbest(preftype="mp4").url  # YouTube URL